The present invention relates to a drive control circuit for a junction field-effect transistor.
Junction field-effect transistors (JFETs), for example, like those disclosed by U. Tietze and Ch. Schenk in Halbleiter-Schaltungstechnik [Semiconductor Circuit Technology], 9th Edition 1990, pages 83 to 101, have the characteristic that they are conducting with a gate voltage of 0 V, but can be pinched off as an n-channel type with negative gate voltage or as a p-channel type with positive gate voltage. However, the required gate voltages vary widely for different samples of same conduction type.
FIG. 1 shows two typical characteristic curves of the gate leakage currents IG for two different junction field-effect transistors. As is shown, the leakage current IG of a junction field-effect transistor JFET1 rises significantly at a voltage U1. In other words, the voltage U1 can only be exceeded slightly at the gate of the JFET1, since otherwise excessive energy dissipation occurs at the gate. In order for the junction field-effect transistor JFET 1 to block reliably, the gate voltage should exceed a voltage U10. Therefore, the gate voltage U10 is only slightly below the gate voltage U1, typically 5 to 10 V. If a fixed gate voltage is selected for all junction field-effect transistors of a converter and is slightly greater than the gate voltage U10 and smaller than the gate voltage U1, a junction field-effect transistor JFET2 is not able to block since the gate voltage is, for example, below its required minimum gate voltage U20. On the other hand, if a gate voltage is selected to exceed the required minimum gate voltage U20 of the field-effect transistor JFET2, the junction field-effect transistor JFET1 would be destroyed by such a drive control since its gate current would rise excessively.
In junction field-effect transistors used in converters, the pinch-off voltages can fluctuate, for example, in a range between −17 V and −57 V. Conventional drive control circuits, which apply the gate-source voltages selectively to 0 V for switching ON and apply a negative voltage for switching OFF, the drive control voltage must be adjusted individually to the respective junction field-effect transistor. This is very complex and unacceptable for mass production.
It would therefore be desirable and advantageous to provide an improved drive control circuit for a junction field-effect transistor to obviate prior art shortcomings.